This invention relates to a method and an apparatus for retreading vehicle tires, and especially such method and apparatus in which microwave energy is utilized for heating a tie-gum layer between the carcass and tread of such tires.
Methods and apparatuses for retreading vehicle tires by utilizing microwave energy are described in U.S. Pat. No. 4,157,930 and in co-pending U.S. patent application Ser. No. 06/453,697 in the name of two of the inventors herein and assigned to the same assignee.
When retreading vehicle tires by utilizing microwave energy, a carcass is provided with a prevulcanized new tread and with a tie-gum layer between the carcass and tread. The tie-gum layer is unvulcanized. Above the tread, in an autoclave, a microwave applicator is provided for generating heat by microwaves primarily in the unvulcanized tie-gum layer. The tie-gum layer is vulcanized when its vulcanization temperature has been achieved and maintained for a certain period, whereafter which retreading is completed.
The contact surface of the tread to the tie-gum layer and the contact surface of the tie-gum layer to the carcass are slightly curved, but relatively flat in the plane which is perpendicular to the radius of the tire. The tie-gum layer normally is slightly wider, as seen in the axial direction of the tire, than the tread. The tie-gum, therefore, projects slightly outside the tire side at the point of vulcanization. The reason for this is to ensure a fully satisfactory vulcanization between carcass, tie-gum layer and tread at the edges of the tread.
Prior to the vulcanization, a so-called envelope is positioned upon the tread which also extends a distance down over the tire sides. The purpose for this is to retain the tread in place relative to the carcass during the vulcanization.
In certain cases there exists a problem when the retreading is carried out in the aforesaid manner, viz. that the vulcanization is not completed satisfactorily at the outer portions of the tie-gum. It was found that the problem is due to too strong a cooling of the tire in the autoclave. The autoclave is not heated when microwave energy is utilized for heating the tie-gum layer. In the autoclave a high pressure, for example 7 bar, prevails, resulting in that the cooling capacity of the air at room temperature is substantially higher than at atmospheric pressure.
The problem, thus, is that with utilization of a microwave applicator for heating the tie-gum layer this layer is heated to the degree desired, except at its outer edges, due to the strong cooling of the tire sides.
One method of heating the lateral edges of the carcass is to position a separate microwave applicator at each lateral edge. This method, however, is expensive and causes aging of the carcass and tread, because they also are heated.
The present invention offers a solution of the aforesaid problems.